1. Field of the Invention
The present invention relates to methods of processing signal samples representative of a colour video image to produce a legalised colour version of the image. Furthermore, the present invention relates to apparatuses for processing signal samples representative of a colour video image to produce a legalised colour version of the image.
2. Description of the Prior Art
It is well known that the colours of the rainbow, which correspond to light with a range of wavelengths which is visible to the human eye, can be represented from combinations of the colours red, green and blue. For this reason colour television and video images are generated by separating the red, green and blue components of the images and sampling these components at spatially separated sampling points within the image. For example, colour television cameras are provided with a dichronic element which separates the colours of an image formed within a field of view of the camera into red, green and blue components. Each of the red, green and blue components of the image is sampled in two dimensions in accordance with a row-by-column de-composition of the image. Each row is sampled at regularly displaced sampling points to produce a number of samples representing the row which produces the row-by-column de-composition of the image. These sampling points are known to those skilled in the art as pixels. Each of the samples represents one of the red, green and blue components of one of the pixels which make up the image.
The colour image may be re-generated from the signal samples using a colour visual display unit, by separating the signal samples representing the red, green and blue components of the pixels and feeding each respectively to one of three image generators. Each of the image generators operates to reconstruct, row-by-column, a version of the image for one of the three colours of red, green or blue which are super-imposed on a colour screen. By producing the red, green and blue components of each pixel at positions on the screen corresponding to the positions of the pixels from which the colour image was sampled, the colour image is re-generated. Since each pixel is comprised of red, green and blue components, the relative intensity of these components produces a mixture of red, green and blue light which represents the colour at the corresponding point of the image. The mixture of the red, green and blue components can therefore reproduce any of the colours of the original colour image, which can be any of the colours of the rainbow. A combined effect of the three image generators is therefore to reproduce a version of the colour image which is representative of the colour image formed within the field of view of the television camera.
Representing a colour image as red, green and blue signal samples provides a facility for transmitting, recording and reproducing the colour image in some way. However, in order to reduce an amount of information which must be transmitted in order to convey the colour image, known television transmission techniques and video image recording techniques convert the red, green and blue signals into colour difference signals, which are generally comprised of a luminance and a first and a second chrominance signal. The luminance signal is, for example, formed by combining the red, green and blue signal components of a pixel into a single component representative of the relative strength of the light in the image at the pixel location. The first of the chrominance signals is generated by forming a difference between the luminance signal and the red signal, and the second chrominance signal is formed from the difference between the luminance signal and the blue colour signal.
The colour difference signal format is one example of a signal format which forms a signal space in which the pixels of a colour video image can be represented, but which does not directly correspond with the red, green and blue components from which the colour video image was generated. As a result, not all values of the colour difference signal components representing a pixel within the colour difference space correspond to pixels within the signal space formed from the red, green and blue components of the colour image. For example, if the luminance component is at its minimum value of zero, then any non-zero value of the two chrominance signal components will result in a signal value which does not fall within the red, green and blue colour reference space. Similarly, if the luminance signal is at a maximum value which corresponds to white light, then any non-zero values of the two chrominance signals will also not fall within the red, green and blue reference space.
Any colour which does not fall within the red, green and blue reference space is an illegal colour. For the example of colour difference signals, any combination of the three components of the colour difference signals which results in a value which does not fall within the red, green and blue colour reference space will be an illegal value. Such illegal colour values can be produced when the colour images are transmitted or processed as, for example, colour difference signals. For example, video signals are often processed in this format to introduce video effects such as colour wash effects. As a result, values of the three colour reference space components can be produced which are illegal values within the red, green and blue reference space. If these illegal colour values are displayed within a colour image, colours can result which do not match with the legal parts of the image. The colour visual display unit reproducing the image may hard limit the colour value to a maximum value of the component which can be displayed, and the illegal pixels may be reproduced or processed in an unpredictable way.
In an article entitled xe2x80x9cLimiting of YUV Digital Video Signalsxe2x80x9d by V G Devereux from the Research Department, Engineering Division, of the British Broadcast Corporation dated December 1987, a method of converting illegal colour pixels in a form of YUV colour difference signals into legal colour pixels with respect to the red, green and blue (RGB) colour reference space is disclosed. This method changes the components of the pixels in the YUV colour difference space with respect to each other in order to convert the pixel in the corresponding red, green and blue colour reference space into a legal pixel.
Having regard to the above discussion, it will be appreciated that there is a general requirement to provide a method of processing colour video images in order to convert reliably illegal colour pixels of the images into legal colour pixels.
According to the present invention, there is provided a method of processing input signal samples representative of at least part of a colour video image to produce legalised signal samples representative of a legal colour version of the image, the method comprising the steps of, generating an over sampled version of the input signal samples by generating at least one extra signal sample for each base input signal sample, generating adjustment factors from the input signal samples, which when combined with the input signal samples have an effect of converting illegal colour pixels of the colour video image into legal colour pixels, combining the adjustment factors with the input signal samples to produce the legalised colour signal samples, selectively filtering the legalised colour signal samples with a filter in dependence upon whether the legalised colour signal samples have changed with respect to the input signal samples and if said legalised colour signal samples have not changed, bypassing the filter, and decimating the filtered legalised colour signal samples to produce a version of the legalised signal samples having a sampling rate corresponding to that of the base input signal samples.
Illegal colours in an image can be produced by high frequency components of the video image in the analogue domain which are outside a maximum frequency which can be represented in accordance with a sampling rate of the input signal samples. These high frequency components produce distortion in the video image as a result of aliasing errors. The distortion produces illegal colours in the image. To provide an improvement by reducing the effect of this distortion, an over-sampled version of the input signal samples is generated so that these out-of-band components in the video image appear within the in-band components of the over-sampled version of the input signal samples. This provides a further advantage in representing the analogue video signal more accurately because the sampling points of the video image at the lower sampling rate can fall at positions which do not correspond to a maximum of the video signal. The over sampled version therefore provides a better representation of the video signal, because there is an increased likelihood that maxima of the video signal are closer to the temporal position of the signal samples. The video image is then legalised in this over sampled form, by generating and applying adjustment factors to produce legalised colour signal samples. However, distortion of the colour video image can occur when an over sampled version of the legalised colour signal samples is filtered with, for example, an anti-aliasing filter, because filtering requires that the signal samples be convolved with the impulse response of the filter. As a result, legal colour pixels or illegal pixels which have been legalised may once again become illegal. Therefore by selectively filtering the legalised colour signal samples in dependence upon whether the legalised colour signal samples have changed with respect to the input signal samples a possibility of making colour pixels illegal as a result of filtering is substantially reduced.
Advantageously, in a preferred embodiment, the step of selectively filtering the over sampled version of said legalised colour signal samples comprises the steps of determining whether each of at least one of the base legalised colour signal samples and the associated extra legalised colour signal samples were changed with respect to the corresponding input signal samples when combined with the adjustment factors, and for each base input signal sample, if at least one of the corresponding input signal samples were changed with respect to the at least one base and associated extra legalised colour signal samples, filtering the at least one base and the associated extra legalised colour signal samples with the filter, or if neither the at least one base and the extra legalised colour signal samples have changed, bypassing the step of filtering the plurality of the legalised colour signal samples and the extra colour signal samples.
As explained above, although the input signal samples which are representative of the colour video image may have values with respect to a signal space which is different from the red, green and blue signal space, an example embodiment of the invention finds particular application where the input signal samples are colour difference signal samples having luminance and two colour difference signal components. As such, in the case where the adjustment factors are calculated and applied with reference to the red, green and blue colour reference space, the step of combining the adjustment factors with the input signal samples comprises the steps of converting the input colour difference signal samples into a colour reference signal samples having values with respect to three orthogonal colour reference axes of red, green and blue light, combining the colour reference signal samples with the adjustment factors and converting the combined colour reference signal samples into colour difference signal samples.
In an alternative embodiment, the step of selectively filtering said over sampled version of said legalised colour signal samples comprises the steps of selectively storing said base legalised colour signal samples from said over sampled version, filtering said over sampled version of said legalised colour signal samples, and following the step of decimating said filtered legalised colour signal samples, determining whether each of said base legalised colour signal samples were changed with respect to the corresponding input signal samples when combined with said adjustment factors, and said corresponding input signal samples has not changed with respect to said base legalised colour signal sample, performing the step of replacing said filtered decimated legalised colour signal sample with the correspondingly selectively stored base legalised colour signal sample from said over sampled version.
In the alternative embodiment, the base legalised colour signal samples are first stored and all the over sampled legalised colour signal samples are filtered with an anti aliasing filter, and decimated. However the decimated filtered legalised colour signal samples are then replaced with the stored base legalised colour signal samples, if the adjustment factors has not had an effect of changing the input signal samples.
Although the adjustment factors may be digital values which are added to the input signal samples in order to generate the legalised colour signal samples, in a preferred embodiment, the adjustment factors are scaling factors and the step of combining the adjustment factors with the input signal samples comprises the step of multiplying the adjustment factors with the input signal samples.
Accordingly to an aspect of the present invention, here is provided an image processing apparatus according to patent claim 7. Further features and aspects of the image processing apparatus are provided in the appended claims.